It is a common procedure for a caregiver to make an image of a patient's body while either planning or performing a surgical procedure. One type of image that is particularly useful is a volume rendering produced from magnetic resonance image (MRI) data, which enables a caregiver to achieve a greater understanding of anatomical shapes and motion. Examples of procedures for using an MRI include angiography for identifying vascular disease, or locating abnormal tissue for possible surgical resection.
A problem with current methods of generating volume renderings from MRI data, however, is that generation of the renderings is slow. In some methods, the MRI scanner generates data for a series of parallel two-dimensional image slices and downloads them to a computer. The computer stores this data and then creates a volume rendering from the two-dimensional slices. One of the problems with this technique is that the volume is rendered off line and thus there is a substantial latency between the time the data is gathered and the time the volume is rendered.
This latency is acceptable for some diagnostic procedures such as an examination to locate and identify abnormal tissue such as cancerous tumors. However, this latency can cause problems in other procedures such as surgical procedures or interventions. An example of such a problem is misregistration between the image and the patient's actual anatomy. Misregistration can occur because of a variety of events such as surgical manipulation or movement of the patient's muscles through normal contraction and relaxation.
One technique to reduce the latency of the volume renderings is to speed up the rendering process itself. This improves user interactivity, such as rotating or translating. It is often referred to as “real-time volume rendering,” but neglects the fact that the image data itself is not real-time, and could have been acquired well before the rendering takes place. Hence, it is not real-time with respect to when the MRI data was acquired.
In other techniques, real-time magnetic resonance (MR) imaging has been achieved, but only when displaying two-dimensional images. These two-dimensional techniques often employ a thin slice to visualize structures in fine detail. However, when manipulating an interventional device, such as a catheter, the tip of the device will frequently move out of the imaging plane as it travels through a patient's body. At this point, the device tip is no longer visible. One way to work around this problem is to continually adjust the position of the imaging plane to include the catheter tip. The caregiver can manually make this adjustment on the computer while viewing the image, but has to continuously and manually adjust the image plane, which is inconvenient and increases the risk of error.
Another way to work around the problem of the catheter tip leaving the image plane is to use a technique called active tip tracking, where a device determines the coordinates of the catheter tip and automatically adjusts the scan plane. However, active tip tracking requires a more complicated catheter design and additional computer hardware or software resources. Not only does this technique increase the complexity and cost of the equipment required for both the catheterization procedure and the MRI system, it can result in a degradation of the image quality if not designed properly.